1. Field of the Invention
This invention relates generally to cardiac rhythm management, and more particularly to a combination of cardiac pacing and optimizing pacing parameter values.
2. Related Art
Dual-chamber pacemakers are used increasingly in patients with varying degrees of heart block, symptomatic bradydysrhythmias, and drug-refractory cardiomyopathy. Clinical benefits of the dual-chamber pacemaker include enhancement of forward blood flow, a feature that can alleviate symptoms of congestive heart failure (CHF), and prevention of atrial fibrillation caused by the atria contracting against a closed valve (Gadler F, Linde C, Darpo B. Modification of atrioventricular conduction as adjunct therapy for pacemaker-treated patients with hypertrophic obstructive cardiomyopathy. Eur Heart J 1998; 19:132-138).
Dual-chamber pacing can improve hemodynamics in some patients with dilated cardiomyopathy, likely by abolishing diastolic mitral regurgitation through the establishment of mechanical atrial and ventricular synchrony (Nishimura R, Hayes D, Holmes D, Tajik A. Mechanism of hemodynamic improvement by dual-chamber pacing for severe left ventricular dysfunction: An acute Doppler and catheterization hemodynamic study. J Am Coil Cardiol 1995; 25:281-288). Despite the benefit of optimization of atrioventricular (AV) delay, dual-chamber pacemakers often are left at the default value, which the manufacturer sets to approximately 170 milliseconds (Kindermann M, Frohlig G, Doerr T, Schieffer H. Optimizing the AV delay in DDD pacemakers with high degree AVE block: Mitral valve Doppler versus impedance cardiography. Pacing Clin Electrophysiol 1997; 20: 2453-2462). It is the consensus of independent researchers that optimization of AV interval is not routinely performed. Procedures for pacemaker optimization, specifically obtaining stroke volume measurements at different AV intervals by aortic Doppler echocardiography, traditionally have been observer-dependent, time-consuming, and costly.
The goal of AV optimization is the synchronization of the completion of end-diastolic filling exactly at the onset of left ventricular contraction. Obviously, to accomplish this objective, precise physiological measurements of the events of the cardiac cycle must be obtained. Because of a wide range of cardiac conditions, status of the ventricles, and cardioactive medications, each and every patient is unique. Leonelli et al. (Leonelli F, Wang K, Youssef M, Brown D. Systolic and diastolic effects of variable atrioventricular delay in patients with pacemakers. Eur Heart J 1995; 15:1431-1440) observed that an optimal setting of the AV delay value improved stroke volume up to 42%.
Another application of cardiac pacemakers has recently been discovered: Recent reports are suggesting that biventricular pacing may offer some important options in the treatment of patients with congestive heart failure (CHF). A significant percentage of patients with CHF have conduction abnormalities on EGG. These conduction abnormalities result in abnormal activation of ventricular myocardium and asynchronous activation of the atrial and ventricular chambers. Biventricular pacing attempts to activate the right and left ventricles simultaneously, producing what is termed “ventricular resynchronization”.
Studies have confirmed acute and short-term hemodynamic benefits of biventricular pacing. In addition, studies have documented improvement in the functional status of patients with CHF. Larger, prospective studies investigating the beneficial effects of biventricular pacing and its clinical implications are currently underway.
In addition to the symptomatic and functional improvements, other important changes have been noted in CHF patients treated with biventricular pacing. Parameters of cardiac function such as left ventricular dimensions and myocardial performance index have improved markedly. Elevated plasma norepinephrine levels, which are associated with increased mortality in CHF, improve in biventricular pacing. Decreased heart rate variability, also associated with increased risk of sudden death in CHF, has been shown to improve. These findings have lead investigators to hypothesize the potential for biventricular pacing to improve survival. This being said, no trial to date has demonstrated a survival benefit to biventricular pacing. Furthermore, no studies are known that investigate the effects, and potential benefits, of biventricular pacing forcing a small delay between right ventricular and left ventricular contraction, or vice versa.
When optimizing the AV delay, or any other delay such as a delay between the contraction of right and left ventricles, it must be tailored to the individual patient. For almost two decades, stroke volume measurements by means of thoracic electrical bioimpedance (TEB) have been favorably considered for optimal determination of pacemaker settings. More recently, Hayes et al. (Hayes D, Hayes S, Hyberger L. Atrioventricular interval optimization technique: Impedance measurements vs Echo/Doppler. Presented at the North American Society for Pacing & Electrophysiology's 19th Annual Scientific Sessions, San Diego, Calif., May 9, 1998) reported that the noninvasive hemodynamic monitoring with TEB permits determination of optimal AV delay within 15 minutes in any clinical settings.
Despite promising benefits to the patient, the utilization of thoracic electrical bioimpedance (TEB), as with any other aforementioned method, has not been established as a standard optimization procedure for the setting of parameter values of dual-chamber pacemakers. Apparently, the TEB procedure, applied during pacemaker follow-up, is time-consuming and requires active involvement of the physician during the entire optimization period.
Rate-responsive cardiac pacemakers address the adaptation of the pacing rate according to the physiological demands related to the activity of the pacemaker patient. Sensors determine, for example, posture and movement of the patient, or respiration, characterized by respiration rate and tidal volume, and even stroke volume by measurement of thoracic electrical bioimpedance. The pacemaker adapts the pacing rate depending on the information obtained by the sensors and processed usually by the pacemaker. The pacemaker's rate adapted to the patient's activity is not within the scope of the aforementioned optimization techniques, and the invention.